1. Field of the Invention
The present invention relates to a photodisc apparatus having a tracking servo means for enabling a beam spot to trace an information recording track with a high degree of accuracy.
2. Description of the Related Art
In recent years, optical information recording/reproduction apparatuses have been put to practical use, in which information is densely recorded in a recording medium by means of a light beam spot focused on the recording medium or information is read or reproduced at high speed through sensing the light reflected from the medium by means of a photosensor.
In the recording/reproduction apparatus of the kind described, it is necessary that the light beam applied to the recording medium is held in a focused state and in an on-track state, in order to record information at a high density and to read information which has been recorded at a high density. To this end, the recording/reproduction apparatus is usually provided with a focus control means and a radial tracking control means. These control means detect, as a focus error signal and a tracking error signal, information concerning an amount of defocus of the beam and radial deviation of the beam contained in the light reflected back from the recording medium.
These control means, however, tend to suffer from fluctuations in the gains of the respective servo loops depending on various factors such as the power of the light source such as a laser diode, reflectivity of the disc, variation in the focus error signal detection system, and so forth. A fluctuation in the loop gain deteriorates the response of the servo loops to focus error and tracking error or causes an oscillation or hunting. Thus, the stability of the servo control is impaired when a change is caused in the loop gain of each servo system.
In order to overcome this problem, Japanese Patent Unexamined Publication No. 63-106003 proposes an art in which a variable gain amplifier is used as an error amplifier, wherein the gain of the variable gain amplifier is controlled in accordance with a gain constant which is set in accordance with a p--p value of an error signal which is obtained in an open state of the servo loop.
In this known art, in order to attain a high accuracy of the peak hold, the peak hold operation is conducted when the rotation speed of a spindle motor has exceeded a predetermined speed while the number of peaks of eccentricity has been increased beyond a predetermined number. It is to be noted, however, that the amplitude of a track error signal (TES) sometimes depends on the speed at which the beam spot crosses the track. The speed at which the beam crosses the track varies depending on various factors such as the amount of eccentricity. This means that the peak hold is not always conducted in an adequate manner.
To explain in more detail, in general, the center of the concentric tracks or a spiral track formed in the surface of a photodisc cannot be precisely aligned with the center of rotation of a spindle motor for driving the photodisc. In some cases, the tracks or track itself has an eccentricity with respect to the center of the disc. Such an eccentricity, particularly when it is large, causes the following problems. Namely, when a light beam is applied to the photodisc through an objective lens while the tracking servo loop is kept open, the beam spot undesirably scans the surface of the disc across the tracks due to eccentricity of the tracks with respect to the center of the spindle motor. The higher the rotation sped of the spindle motor, the greater the number of the eccentricity peaks. Conversely, the number of peaks is reduced when the rotation speed of the spindle motor is lowered. It is also to be understood that the influence of the degree of eccentricity of the spindle motor varies in each rotation of the spindle motor. Namely, the influence is large, small or zero at different rotational phases in one rotation. A tracking error signal of a waveform as shown in FIG. 1 is obtained when the tracking servo is kept off, even when an eccentricity exists.
Referring to FIG. 1, many peaks of tracking error signal are obtained in a period a. In this period, therefore, the influence of any noise on the positive and negative peak values in each short period of the tracking error signal is sufficiently small. On the other hand, in a period b, the peak values are influenced more seriously because the signal levels around the peaks are maintained for a longer time than in the period a.
The known art mentioned before does not take the eccentricity amount into consideration. Namely, the peak values of the tracking error signal are liable to be influenced by the noise, so that the accuracy of holding the peaks of the tracking error signal is impaired particularly when the eccentricity of the track in the disc is small, even when the rotation speed is enhanced.
Furthermore, an error in the peak hold tends to occur in the period in which the amount of eccentricity is small within one full rotation of the photodisc.
It is also to be pointed out that a considerably long waiting time is necessary because the detection of the peak value is conducted after the disc rotation speed has reached a predetermined level.